Bleaching alkyd resins



iii-? AYE;

l "i eiereiite fig R 2,107,6lt

BLEACHING ALKYD RESINS Almon G. Hovey, Pleasant Ridge, Mich, assignor toHelmuth lteichhold, doing business as Reichhold Chemicals, Detroit,Mich.

No Drawing. Application June 14, 1937, Serial No. 148,204

i Claims.

The invention relates to bleaching alkyd resins. Although alkyd resinsare one of the lightest colored resins now in use for enamel vehicles,nevertheless, it is commercially desirable to have them even lighter incolor than they-are. I have discovered that it is possible to bleachalkyd resins to a very desirable degree by incorporating a small amountof bleaching agent directly into the batch before thinning. Thebleaching agents which appear to be most successful are the threematerials: citric, tartaric and oxalic acids.

While it is true that oxalic acid has been used as a general bleachingagent for such purposes as straw hats and for removing stains (U. S.Patent No. 1,032,229, Melchers, 1912), nevertheless it is impossible touse such materials as a bleaching agent for alkyd resins simply byincorporating them into the enamel. First of all, citric, tartaric andoxalic acids are water soluble materials and do not lend themselvesconveniently to solution in such solvents as are customarily used in thecommercial alkyd resins, such as toluol, xylol, high flash naphtha,hydrogenated naphtha or mineral spirits. While a certain amount of theseacids might be incorporated by physical addition, such as grinding inwith the pigment in a mill into the finished enamel, neverthelessexperience has shown that this is not a satisfactory method because thedispersion of the bleaching substances is very imperfect. Secondly, anenamel consisting of pigment, solvent and alkyd resin in which thebleaching agent is added mechanically during the formation of theenamel, shows the disagreeable effect of cratering or surfaceirregularity due to the fact that in the baking of the enamel thebleaching agent reacts with free hydroxyl groups in the alkyd,liberating minute quantities of water. These minute quantities of watercause variations in surface tension which produce the irregularity inthe enamel film.

I have found that when an alkyd resin is prepared the color may beconsiderably lightened if a very small amount (between 0.1% to 0.5%) ofcitric, tartaric or oxalic acid is added at the high temperature atwhich the resin is ordinarily cooked, just a short time before the resinis to be thinned. In this way, the small amount of the acid bleachingagent can accomplish the bleaching effect, react with the free hydroxylgroups, liberate the Water vapor and become an integral part of theresin mass without destroying any of the good properties of the enamel.In this way, no water-soluble material is present in the enam" e1 whichmight cause instability of the goods in i. her/Mg.

the package during storage. Since the water vapor has been driven offalready by the heating there will be no Water driven off in the finalenamel, which can cause irregularity of surface, yet, however, the muchdesired bleaching effect is accomplished. Depending upon how muchbleaching agent is used, the color may be lightened as much as one totwo full shades on the Hellige-Klett color scale for determining theoriginal color on commercial alkyd resin solutions, thus rendering theproduct much more attractive for sale. Furthermore, the color of theenamel film after baking is vastly improved over that of an enamel whichhas been untreated.

Emample 1 Per cent Phthalic anhydride 38.6 C. P. glycerine 25.4 Soyabean oil fatty acid 36.0

Total 100.0

These materals are heated together at 230- 240 C. until a viscosity of Mon the Gardner- Holdt scale is obtained for a 50% solution in xylol. Atthis point citric acid in the amount of one-fourth of one percent byweight of the total amount of resin ingredients is introduced. A certainamount of foaming takes place but not to any dangerous extent. The coloris immediately bleached. After allowing the reaction to take effect andthe vapors to evaporate (between one-half to three-fourths of an hour)the resin is withdrawn and thinned to a viscosity of approximately 0 onthe Gardner-Holdt scale at a concentration of 50% in xylol.

Example 2 Per cent Phthalic anhydride 38.50 Glycerine 26.98 Linseedfatty acid 34.52

Total 100.00

In both these examples it is understood that the resin should be cookedunder an inert atmosphere and that all the other usual precautions tokeep the color light should be taken. In the bleaching process it isimportant that the bleaching operation be performed as near to the endof the cooking process as possible in order that the color may notreturn. The resin, if

leached at the start of the cook, instead of at the end, regainspractically all the color which the bleaching agent has taken away andsometimes even ends up darker than it would have been had there been nobleaching agent introduced. Thus it is about as bad to bleach too soonas if it had never been bleached at all.

The use of this very small amount of polybasic acid as a bleaching agentis not to be confused with any processes known to the art wherein arelatively high amount of citric and tartaric acids are used as part ofthe main resin-forming ingredients. Resins which have been so describedare invariably very poor on account of the lack of water resistance. Inusing this almost infinitesimally small amount of the agents, thebleaching action is accomplished without any of these deleteriouseffects when humidity and water resistance tests are made on thefinished enamel. It is well known that alkyd resins cannot be producedfrom oxalic acid and glycerine at a temperature of 230-240 C. becausethe oxalic acid decomposes. It has been possible to form a resin fromoxalic acid and glycerine if the temperature is kept very low for longperiods of time, such as 24-72 hours, but this is not in accordance withpractice which I have just discussed. Other hydroxy acids than citric ortartaric may be employed as bleaching agents, but the acids hereinmentioned appear to be the best for the purpose because of availabilityand relatively low cost.

I claim:-

1. A process of bleaching alkyd resins which comprises incorporating inthe resin during the course of manufacture 01-05% by weight, based onthe total amount of resin ingredients, of an acid which is a member of agroup consisting of citric, tartaric and oxalic, at a time long enoughbefore the batch is thinned so that the products of reaction may escapeand the bleaching effect take place, and at a temperature substantiallyabove 200 C., the bleaching agent being added near the end of thecooking process.

2. A process as set forth in claim 1, wherein the bleaching agent iscitric acid.

3. A process as set forth in claim 1, wherein the bleaching agent istartaric acid.

4. A process as set forth in claim 1, wherein the bleaching agent isoxalic acid.

ALMON G. HOVEY.

